Hymenoptera

I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars.”       – Charles Darwin.

The Hymenoptera are the 3rd largest order of insects (<150,000 species described) and comprise the bees, wasps, ants and sawflies. Hymenoptera are an incredibly abundant, ubiquitous and diverse order of insects and can be identified by their ovipositor or stinger, presence of hooks (hamuli) clipping together forewings and hindwings and a constricted waist (except in sawflies) known as the petiole.

IMPORTANCE. If the diptera order of insects is regarded as the most harmful to man, then the hymenoptera must be considered as the most beneficial. Members of the hymenoptera clear up waste and organic matter (predatory wasps), breakdown deadwood (ants), parasitise and kill pests of agriculture and forestry (parasitoid wasps) and pollinate the majority of wild and commercially grown crop plants (bees). It is by exploiting the parasitic niche that wasps have diversified into species that can attack (and live off) different life stages of all other insects, making there possibly more species of hymenoptera than any other order of insects.

CLASSIFICATION. The order is divided in 2 suborders: Symphyta (sawflies) and Apocrita, which contains the wasps, ants and bees. Apocrita can be further divided into the subclades Parasitica (75% of total hymenoptera) and Aculeata, which contains important superfamilies such as the apoidea (bees), the vespoidea (social wasps) and the formicoidea (ants).

IDENTIFICATION. Symphyta can be distinguished by not having a constricted waist, and possession of a saw-like ovipositor. In the Apocrita however, the first segment of the abdomen (propodeum) is fused to the thorax, forming a mesosoma in bees and a alitrunk in ants. The swollen remainder of the abdomen (attached to the body by the petiole) known as the gaster, is very characteristic of aprocritan wasps. Within Apocrita, Parasiticans possess slender (and sometimes long) ovipositors for piercing and laying eggs in insects or plant tissue. Aculeates however have their ovipositor modified for defence (a sting). Larvae of hymenoptera are either eruciform (caterpillar-like), like in the Symphyta, or grub-like (vermiform) as in the Apocrita.

A free key is available here

REPRODUCTION. Most hymenoptera (especially those forming social castes) reproduce using a haplodiploidy sex determination system. This means that females (including the queen) develop from fertilized eggs with 32 chromosomes (diploid) whereas male drones are produced from unfertilized eggs and only have 16 chromosomes (haploid). Therefore when a queen and drone produce a diploid female worker, that worker is actually more related to her sisters than she is to either parent  (as workers always receive the same 16 chromosomes from their father, and half of the mother’s 32 chromosomes). Because a female worker has a relatedness of 0.75 with their sisters as opposed to 0.5 with any offspring they may produce, workers in social castes of bees, ants or wasps will conserve more of their genes by helping to raise their parent’s young (the brood) instead of reproducing themselves. This fact is key in explaining the evolution of eusociality in many hymenopterans.

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